As the CustomCondition skips undefined/unparsable dates this allows you to let the list owner define his vacation date range without modifying the scenario so you can lock the scenario but still let owners specify a vacation when needed !

Here is the trick, the do_it at the end of this rule doesn't matter as the custom condition will never say it matched, this allows to use a custom condition as some kind of “message listener” !

You can easily tweak the vacation message on a per list basis if needed.

You can also specify several vacation date ranges, just add as much versions of the rule as needed !